1. Field of the Invention
The present invention relates to a light-emitting device such as, for example, an organic light-emitting device. It particularly relates to an organic light-emitting device that is able to emit light having a degree of polarisation. The invention may be applied to an organic electroluminescent device (OLED).
2. Description of the Related Art
Many OLEDs are known, but currently available OLEDs generally emit light that is substantially unpolarised. It would be desirable to manufacture an OLED that is able to emit polarised light, for example plane-polarised light. (The term “polarised light” as used herein includes light that is partially polarised, and is not limited to light that is completely polarised.)
For example, one use of an OLED is as a light source for a liquid crystal (LC) display device. A transmissive liquid crystal display device consists essentially of a layer of addressable liquid crystal material disposed between a front polariser and a rear polariser. The display device is illuminated by a light source which is disposed behind the rear polariser and so is known as a “backlight”. An OLED that emits polarised light would be particularly useful for a backlight for a conventional liquid crystal display device, because it could make the rear polariser unnecessary and so would allow the rear polariser, and its associated cost, weight and power-inefficient absorptive loss, to be eliminated.
An OLED contains a light-emissive layer, which emits light by the phenomenon of electroluminescence. Polarised electroluminescence (EL) has been achieved by aligning the molecules of the emissive layer within the device and several approaches have been used to achieve this alignment as described by M. Grell et al. in “Adv. Mat.” 11(11) pp895-905 (1999). The degree of polarisation of the emitted light can be expressed as the “dichroic ratio” (D), which is defined as the ratio of the intensity of the emitted light polarised parallel to the alignment direction of the emitter molecules to the intensity of the emitted light polarised perpendicular to the alignment direction of the emitter molecules. Depending upon the intended use of the OLED, the dichroic ratio is normally required to be in the range D≈12-200.
One conventional approach obtaining polarised electroluminescence is to align the molecules of the emitter layer by stretching the emitter layer. Polarised electroluminescence, with D≈2.4, has been reported for a stretched polythiophene film by P. Dyreklev et al., in “Adv. Mat.” 7(1) pp43-4.5 (1995). However, with this method it is not possible to set an exact thickness for the stretched light-emissive layer. Furthermore, inhomogeneities and cracks can occur in the emissive layer as a result of the mechanical stretching process.
A different prior approach is that of rubbing a conjugated polymer film to define a preferred alignment direction for molecules of the polymer film. M. Hamaguchi et al. have reported, in “Appl. Phys. Lett.” 67, pp3381 (1995), photoluminescence (PL) with a dichroic ratio D≈5 for alkoxy substituted poly(phenylenevinylene) (NO-PPV). However, the rubbing process leads to scratches in the polymer film, and these shorten the lifetime of the device.
Another method for aligning molecules of an emitter layer is that of Langmuir Blodgett (LB) deposition. G. Wegner describes, in “Thin Solid Films” 216, 105 (1992), stiff-chain polymers having solubilising alkyl side chains, such as phthalocyanines or poly(p-phenylenes), as a new class of substances which form LB films. In such polymer films, the polymer molecules are aligned parallel to the substrate and display anisotropic behaviour in respect of various physical properties. The surfaces of such LB films are very homogeneous and the thickness of the layers can be set very accurately. EP 0 081 581 describes EL with D≈4 for a device fabricated by direct build up conjugate stiff chain polymers using the LB technique. The LB technique is however unsuitable for device manufacture on a commercial scale.
Many organic EL materials possess liquid crystalline (LC) phases. LC self-organisation is a very effective method of achieving high degrees of alignment, because of the high order parameters of liquid crystal materials. The EL material can be either a low molecular weight (LMW) LC material where hole-conducting or electron-conducting groups, for example, are attached to a LC backbone or an LC polymer such as one of the substituted poly-phenylenevinylenes (PPVs) or one of the polyfluorenes (which are well-characterised EL polymers).
A specific problem for polarised electroluminescent devices based on liquid crystal-material is that such devices require an alignment layer to align the liquid crystal material, and this applies to both LMW and polymeric LC materials. Conventional alignment layers, such as polyimides, are electrical insulators and would therefore strongly hinder the device performance. G. Luissem et al. describe, in “Liquid Crystals” 21(6) pp903-907 (1996), the alignment of LC polymers consisting of isolated phenylenevinylene units separated by flexible spacers on a rubbed polyimide alignment layer. The described current-voltage characteristics show that the device turn-on voltage is around 15V, which is too high for the device to be of practical use. Furthermore, the observed dichroic ratio D≈7 lies below that typically required.
U.S. Pat. No. 5,748,271 describes a layer that combines alignment of an LC material with another device function. For example, rubbed layers of the hole injection material poly(3,4-ethylenedioxythiophene) (PEDOT) or polyanilene (PANI) have been shown to align reactive mesogen/perylene blends with a dichroic ratio D≈2. However, LMW LCs can be aligned more easily than polymers, so it is not at all certain that rubbed PEDOT or PANI will align a conjugated LC polymer.
Moderate alignment of LC polymers (copolymers of polyfluorene) was obtained by M. Grell et al., in “Liquid Crystals” 26(9) pp1403-1407 (1999) using a rubbed film of the precursor of PPV in conjunction with an active material that emits at a wavelength far enough towards the red end of the spectrum (λ>550 nm) for precursor PPV to appear transparent to the emitted light. Anisotropy in the absorption was observed with a maximum dichroic ratio D≈10. Photoluminescence or electroluminescence were not recorded. Generally, alignment using rubbed active layers is not particularly good and the anisotropy possible is much less than for a standard rubbed polyimide alignment layer.
One particular application for light emitting elements is in a stereoscopic display device. A stereoscopic display, where the viewer has to wear, for example, polarising or colour-filter glasses to obtain a 3D image effect, are well-known, and are described in, for example, “Stereo computer graphics and other true 3D technologies” edited by D. F. McAllister, pp90-115, Princeton University Press (1993). Essentially, a stereoscopic display device displays an image for the left eye of an observer and another image for the right eye of the observer. The two images may be displayed time-sequentially, or they may be displayed simultaneously on different parts of the display. The two images are in different wavelength ranges or have different polarisations, and the observer must wear polarising or colour-filter glasses that allow the observer's left eye to see only the left eye image while allowing the observer's right eye to see only the right eye image. The use of images having different polarisations is today more common.
A stereoscopic display device may be a passive device, in which the polarisers or colour filters provided on the display device and the polarisers or colour filters in the observer's glasses are passive devices—i.e., they have fixed transmission properties such as fixed transmission axes (in the case of polarisers) or transmit over fixed wavelength ranges (in the case of colour filters).
Alternatively, a stereoscopic display device may be an active device, in which the transmission properties of the polarisers or colour filters provided in the observer's glasses are controllable and so can be changed.
Stereoscopic displays often use a conventional spatial light modulator (SLM), such as a liquid crystal display device, with additional internal or external optics attached to the display to generate the initial conditions for stereoscopic viewing. These additional optics can add to the bulk, complexity and cost of the display device, and often cause display contrast degradation (for example where neighbouring columns in the SLM are operating in normally black and normally white mode as disclosed in U.S. Pat. No. 5,264,964).
Where the image forming element is a conventional cathode ray tube (CRT) with either active or passive polarising optics attached to the face-plate, the resulting 3-D image often suffers from high cross-talk. Cross-talk is the ratio of one eye image perceived by the second eye, and results in this case from CRT phosphor persistence as described by D. F. McAllister (above). This problem occurs, for example, when the left eye image and right eye image are displayed in a time-sequential manner. Further, there is often a perceived brightness variation down the CRT screen caused by low liquid crystal switching speeds in the polarising optics and CRT phosphor persistence. It is generally desirable for a stereoscopic display device to be able to function as a conventional 2-D display device. This can be done, irrespective of whether a LCD or CRT is used as the image-forming element, by displaying conventional 2-D images and having the user remove their polarising or colour-filter glasses. However, conventional stereoscopic viewing systems require the provision of the external permanent polarising optics on the display, and the polarising optics normally reduces the inherent brightness of the display. For example, in a device having a CRT display screen the CRT phosphors emit nominally unpolarised light, and polarising optics are mounted on the face-plate to polarise the light emitted by the CRT phosphors. The inherent brightness of the CRT display is reduced owing to absorption of light in the polarising optics.
One prior art approach to the problems of variations in brightness down the screen and high display cross-talk, for a passive stereoscopic display system, is to partition the liquid crystal switch attached to the image forming element. However, this approach adds cost and complexity to the LC switch drive circuitry. Furthermore, this solution cannot be adopted for active stereoscopic systems, as described D. F. McAllister (above).
As discussed above, high cross-talk and brightness variations down the screen are often perceived with CRT-based stereoscopic systems owing to image persistence resulting from the long radiative decay times of the CRT phosphors. This problem can be avoided when using emissive displays with shorter radiative decay times. One example of this type of display is organic light emitting diodes (OLEDs).
It has been suggested, in “Advanced Materials”, Vol. 14(20), pp1477-1480 (2002) to use a conventional unpolarised OLED as the display screen in a passive stereoscopic 3D display system, working in co-operation with a permanent patterned polariser. This addresses the problem of image persistence, but has the disadvantage that the inherent brightness of the OLED display is significantly reduced due to the analysing affect of the patterned polariser.
At best, 50% of the inherent display brightness would be transmitted through the patterned polariser. Increasing the OLED current density can increase the intensity of the output from the OLED and compensate for the intensity loss caused by the polariser, but this severely degrades the OLED lifetime. Further, this prior art suffers from the need for correct registration between the patterned polariser and the pixels of the OLED display. Without correct registration, Moire effects would be observed (degrading the image quality in display of a 2-D image) and parallax problems would result in high cross-talk when the device is used to display a 3D image. The need for correct registration means that the patterned polariser and the OLED display must be manufactured to strict tolerances, thereby increasing the manufacturing costs.
To avoid the decrease in inherent display brightness, it would be advantageous if emissive displays with short radiative relaxation times could be used, where the light emission from the image-forming display elements was inherently polarised (with a high polarisation purity). This would avoid the brightness reduction, cost, complexity and bulk of adding external polarising optics.
Methods to achieved polarised emission from an aligned conjugated polymer (that could be used for an OLED display) have been disclosed in a conventional 2-D display in Chem. Phys. Lett. 341, pp219-224 (2001). This prior art used a highly birefringent emissive polymer sandwiched between a dielectric Bragg-stack mirror and a metallic mirror in a microcavity. The birefringence of the polymer affects the resonance modes of the cavity, allowing one polarisation to resonate constructively within the cavity whereas the modes of the other polarisation destructively interfere. Although not demonstrated for electrically driven light emission, photo-excitation showed that polarisation purity ratios exceeding 300:1 were obtained. However, the use of microcavities is complex and costly, they suppress the inherent brightness of the OLED (and thus affect OLED lifetime owing to the need to increase the drive current to compensate), and they are expected to show significant colour and brightness variations when viewed in directions away from normal incidence. Further, there may also be light leakage of the wrong polarisation state at a different colour during emission of the desired polarisation state. This would lead to cross-talk and colour artefacts if used in stereoscopic display device operating in the 3-D mode, and the colour artefacts would also be present when operating in the 2D mode.
GB-A-2 344 691 discloses an electroluminescent device in which the emitter molecules of the light-emissive layer are aligned along a pre-determined direction so that the emissive layer emits polarised light. The emissive molecules are aligned either by applying an external electric or magnetic field and fixing the direction of the emissive molecules while the external field is applied or by a separate alignment layer provided in the device specifically to align the molecules of the emissive layer.
EP-A-1 081 774 discloses an organic light-emitting device in which a conventional rubbed alignment layer is provided in the device specifically to align the molecules of the emissive layer thereby to obtain emission of polarised light.